Operating parameter recorders for transportation equipment are usually installed to provide owners with supervisory control through the information provided on the recorders over the operation of the equipment. Efficient operation requires that certain operating parameters such as driving speed, engine rpm, etc. be kept within certain limits which are controlled by the driver, pilot or crew of the equipment.
The most popular types of recorders are ones which visibly record the parameters in analogue fashion on tapes or circular recording disks. For practical purposes, such as the insertion and removal of the recording medium, these recorders must be accessible to the operating personnel. However, access to the recording medium also allows access to the recording mechanism including the stylus which actually records the information on the medium. Accordingly, the operating personnel are able to tamper with the recording mechanism such that the recordings are not accurate. For example, it is highly desirable that transportation equipment such as large trucks and the like be operated at safe and economical speeds however the operating personnel for various reasons often like to run the equipment above these safe and economical speeds. The personnel therefore readjust the stylus such that the analogue values shown on the recording medium appear as being lower than the actual values. The type of adjustment or tampering done to the recording medium depends on the design of the particular recorder. For example, the operating personnel may bend the recording stylus downwardly or they may attach rubber bands, additional springs, etc. to limit the travel of the stylus for altering the recorded value for the operating parameters.
Although these tampering methods can be depicted by trained chart interpreters who are familiar with the characteristics of a particular recorder detection still requires more than just a casual glance analysis of the recordings. Furthermore, untrained interpreters may not be able to recognize an altered recording pattern since there is very little on the recording medium itself to indicate tampering to the recording stylus.